Synergistic, aqueous PAH degradation by ultrasonically-activated persulfate depends on bulk temperature and physicochemical parameters

摘要

Coupling ultrasound with other remediation technologies has potential to result in synergistic degradation of contaminants. In this work, we evaluated synergisms from adding high-power ultrasound (20 kHz; 250 W) to activated persulfate over a range of bulk temperatures (20-60 degrees C). We studied the aqueous degradation kinetics of three polycyclic aromatic hydrocarbons (PAHs: naphthalene, phenanthrene, and fluoranthene) treated by ultrasound-alone, heat-activated persulfate, and combined ultrasonically-activated persulfate (US-PS). At 20 degrees C, observed US-PS rate constants strongly correlated with Wilke-Chang diffusion coefficients. This correlation indicates PAH molecules diffuse to the bubble-water interface prior to reaction with sulfate radicals (SO4 center dot-) generated at the interface. At higher temperatures, observed US-PS rate constants appear to be a more complicated function of temperature and diffusion coefficients. Synergy indexes for PAHs with fast diffusion coefficients were greatest at 20 degrees C. Fluoranthene, the largest and most hydrophobic PAH, had a maximum synergy index at 30 degrees C; it benefited from additional thermal persulfate activation in bulk solution. Fluoranthene synergy indexes, however, decreased above 30 degrees C and became antagonistic at 60 degrees C. Electron paramagnetic resonance (EPR) spin trapping was used to quantify hydroxyl radical ((OH)-O-center dot) produced from acoustic cavitation in the absence of persulfate. These data showed consistent (OH)-O-center dot production from 20 to 60 degrees C, indicating PAH antagonisms at 60 degrees C were not due to lower bubble collapse temperatures. Instead, the results suggest that PAH antagonisms are caused by increased radical-radical recombination as bulk temperature increases. In effort to develop an efficient, combined remediation technology, this work suggests bulk temperatures between 20 and 40 degrees C maximize US-PS synergisms.